The word dream has two meanings:

One meaning is for the dreams while are asleep "I had a bad dream last night"

and the other meaning is about your hopes for the future. such as  " whats your hopes and dreams for the future ?" dreams in this sense is usually plural, since most people have many ambitions and hopes.

Your sentence is I think about the second, as it is unlikely you would dream about a lover before meeting him.  

Another thing to note - you can also say "find your dream man", but it doesn't use plural. That's because that is the dream man! The man is the dream, the dream is the man. It's all one.

But when you say "one of", you can NOT have "one of ONE" or "one of SOMETHING". You can only have one (or more) of multiple choices. 

I am kinda stupified by this. Because! :D How do I answer what i learned naturally?

Okay, let me try.

You. Right? You have dreams. Many dreams. Not one. Not just one like Hitler. Many! So one of your dreams, many dreams, is that you want to find a specific man. When you find the man (according to your dream), this is what actually happens, you find the man of your dreams - the one you imagined and hoped to find.

 Basically, just forget it all and learn the phrase without any questions!